Main iron ores used in steel plants are uniform powdery iron ore made by blending powdered iron ores of different origins/proveniences and different properties. When powdery iron ore is directly loaded into a shaft furnace, the shaft furnace is clogged by the powdery iron ore thus inhibiting the flow of a reduction gas. Therefore, in general, small amounts of lime powder and coke are mixed with powdery iron ore in advance, and the powdery iron ore thus mixed with the lime powder and the coke is sintered in a sintering machine into sintered ore pellets of a certain size. In Japan, sintered ores currently account for substantially 75% of the iron ores loaded into a shaft furnace.
The sintered ore is manufactured according to the following steps. Firstly, a granulated sintering material, which is formed by mixing ore powder, lime powder and coke powder, is loaded into a sintering machine and burned. Then, while the sintering material is moved by a conveyor toward a terminal portion, the coke powder is burned by air blown from above to below, which was sucked by a suction blower. Thereafter, the ore powder is partially melted by a combustion heat of the coke so as to bond. Then, the bonded ore is fragmentized and selected, so that sintered ore pellets having a diameter of 15 mm to 30 mm are obtained. The high temperature sintered ore manufactured in the sintering machine is transferred to a sintered-ore cooler. While being transported by a conveyor, the sintered ore is subjected to a cooling air from below the conveyor, so that the sintered ore is cooled to a temperature at which the sintered ore can be stored.
As described above, the sintering facility for manufacturing sintered ore is composed of the sintering machine and the sintered-ore cooler. The sintering machine is configured to burn a sintering material by supplying air thereto. In this case, a gas generated by the combustion becomes an exhaust gas which temperature ranges from a low temperature of about 50° C. to 60° C. at an ignition portion, to a high temperature of about 400° C. to 450° C. at a terminal portion of a conveyor. The sintered-ore cooler is configured to use air to cool the high-temperature sintered ore. In this case, the cooling air becomes a high-temperature exhaust gas of 300° C. to 400° C.
Conventionally, as shown in FIG. 6, for example, a surplus heat of an exhaust gas of a sintered-ore cooler 2 is recovered by a waste heat boiler 30, and steam is generated by the heat. The steam is utilized as a utility steam or as power obtained through a steam turbine 51. In this way, the waste heat of the exhaust gas from the sintered-ore cooler 2 is effectively recovered.
Patent Document 1 discloses an improved invention related to a waste heat recovery method in which a cooling air heated by the sintered-ore cooler 2 is introduced into the waste heat boiler 30 to generate steam, and the generated steam is supplied to the steam turbine 51 so as to generate power.
In the method of recovering a waste heat of the sintered-ore cooler 2 disclosed in Patent Document 1, the sintered-ore cooler 2 is divided into a boiler communication area in which a sintered ore has relatively a higher temperature, and a gas-duct communication area in which a sintered ore has a lower temperature. A cooling gas introduced into the boiler communication area cools a sintered ore, and is then introduced to a boiler through a hood covering the sintered ore, so that a heat of the cooling gas is recovered. A cooling gas introduced into the gas-duct communication area is directly introduced to a gas duct so as to be released to an outside atmospheric air. A feature of the method disclosed in Patent Document 1 resides in that an inside of the hood is always kept at a positive pressure so that an outside atmospheric air cannot enter thereinto in order to prevent lowering of the temperature of a cooling gas to be recovered, and that a partition between the boiler communication area and the gas-duct communication area can be optionally set, whereby a heat recovery rate can be improved.
Patent Document 1 neither describes nor suggests that a surplus heat generated in the sintering machine 1 is recovered and used.
A cement calcination plant including a suspension preheater (PH) and an air quenching cooler (AQC) conventionally uses a waste heat power generation system in which a heat of an exhaust gas of the PH is recovered by a boiler to dry a cement material, and a heat of an exhaust gas of the AQC is recovered to the maximum extent by a boiler so as to generate power. A temperature of the exhaust gas of the pH is 350° C. to 400° C., for example, and a temperature of the exhaust gas of the AQC is 300° C. to 250° C., for example. An amount of the exhaust gas of the AQC is generally greater than that of the PH.
Citation 2, for example, discloses a waste heat power generation system of a cement calcination plant, in which a waste heat of a PH and a waste heat of an AQC are respectively recovered by waste heat boilers, steam is obtained, and power is generated by a turbine driven by steam.
In the waste heat power generation system of the cement calcination plant disclosed in Citation 2, a part of hot water heated by an economizer of an AQC boiler 130 is transformed into low-pressure steam through a flasher, and the low-pressure steam is introduced into a low-pressure stage of the steam turbine. In addition, a part of the remaining hot water is superheated through an evaporator and a superheater of the AQC boiler 130, and the remaining part is further superheated through an evaporator and a superheater of a pH boiler 110. Then, a generated high-pressure steam is introduced into a high-pressure stage of the steam turbine.
As shown in FIG. 7, a feature of the waste heat power generation system of Patent Document 2 resides in that a second evaporator including a steam drum is further provided on an exhaust-gas exit side of the PH boiler 110, that hot water returned from the flasher is introduced into the second evaporator through the steam drum, that the hot water heated by the second evaporator is returned to the steam drum, and that steam generated by the steam drum is put into the low-pressure stage of the steam turbine.
The disclosed waste heat power generation system maintains an outlet gas temperature of the AQC boiler 130 as low as possible. In addition, the PH boiler is configured to generate high-pressure steam and low-pressure steam, so that steam suitable for the respective high-pressure stage and the low-pressure stage of the multi-stage steam turbine can be supplied thereto. Thus, an outlet gas temperature of the PH boiler 110 can be maintained as low as possible. The above system is designed to significantly improve a waste heat recovery rate.
In the disclosed system, a gas temperature, which is 325° C. at an inlet of the PH boiler 110, is lowered down to 165° C. at an outlet thereof. On the other hand, a gas temperature, which is 360° C. at an inlet of the AQC boiler 130, is lowered down to 105° C. at an outlet thereof.
Namely, the disclosed waste heat power generation system can sufficiently recover a waste heat of the AQC and can sufficiently use a waste heat of the PH, so as to transform the waste heat to electrical energy.
From the above, it is conceivable that applying the technical idea of a waste heat power generation system of a cement calcination plant to a sintering facility including a sintering machine and a sintered-ore cooler enables the effective use of a waste heat of the sintering machine. In this case, the sintering machine is combined with a sintering machine boiler (SM boiler), which corresponds to a PH boiler, and the sintered-ore cooler is combined with a sintered-ore-cooler waste heat boiler (SC boiler), which corresponds to an AQC boiler.
However, in the sintering machine, a sulfur component contained in a sintering material is oxidized in the course of a sintering process to generate sulfurous acid gas SO2. In addition, sulfuric anhydride SO3 is generated by further oxidation. Thus, sulfuric anhydride SO3 is contained in an exhaust gas. Therefore, when a temperature of the exhaust gas becomes lower than an acid dew point, the sulfuric acid gas formed by SO3 reacting with steam might condense (form dews) causing sulfuric acid drops to appear on solid surfaces and display the highly corrosive nature of the sulfuric acid drops. Thus, there is a possibility that an outlet portion of the sintered-ore-cooler waste heat boiler, an exhaust gas treatment apparatus provided on a passage through which an exhaust gas of the sintering machine flows, a gas duct and so on might be corroded and damaged.
In the waste heat boiler disclosed in Patent Document 2, an exhaust temperature at an outlet thereof is lowered to thereby obtain an effective waste heat recovery. Thus, when the sintering machine and the waste heat boiler are combined based on the technical idea of the waste heat power generation system of the cement calcination plant disclosed in Patent Document 2, an exhaust gas temperature condition required for the sintering-machine waste heat boiler cannot be satisfied. For example, the exhaust gas may be excessively cooled, causing the outlet portion of the sintered-ore-cooler waste heat boiler and an exhaust gas system to be damaged. Thus, the technical idea of the waste heat power generation system of the cement calcination plant cannot be applied directly to the sintering facility. Thus, a waste heat of an exhaust gas of the sintering machine cannot be effectively recovered in conventional cases.
Patent Document 1: JP2000-226618A
Patent Document 2: JP2008-157183A